When fine structures are being examined, it is important to protect the preparation from scattered light, which otherwise decreases contrast and washes out fine detail. In microscopy, scattered light is excluded if only the visible field of view in the preparation is illuminated, and the rest remains dark. This function is performed by the field diaphragm. The aperture diaphragm, on the other hand, substantially determines the imaging properties, which can be modified in favor of contrast or resolution depending on the requirements of the various preparations. Both diaphragms are therefore critically important in terms of the quality of the microscopic image, and must be individually adjusted for the particular magnification and specimen conditions.
On the microscope, the objective nosepiece is usually fitted with a plurality of objectives. The selection extends from a scanning objective to objectives having very high magnification and resolution. To ensure that the field of view can be changed quickly while using the microscope, however, it is also necessary to adapt the illumination in each case. This is achieved by adjusting the field and aperture diaphragms. The requirements of the objectives in terms of illumination—regarding the object field to be illuminated, and the illumination angle—are sufficiently different, however, that adaptation of the geometric flux within the illumination system is necessary, since the size and the emission characteristics of the light sources used are limited. This adaptation of the geometric flux of the illumination is achieved by switching different condensers into the beam path for the individual magnification ranges.
In ordinary microscope transmitted-light illumination systems, the aperture and field diaphragms are located in the stand base. As depicted in FIG. 1 (existing art), this type of illumination ensures that the aperture diaphragm is imaged into the entrance pupil of the condensers (FIG. 1A), and the field diaphragm is imaged into the preparation plane (FIG. 1B). In such a configuration, the illumination source is usually imaged into the aperture diaphragm plane; this is referred to as “Köhler illumination.”
In order to illuminate the entire objective magnification range from 1× to 100×, the existing art describes different condenser heads or complex condenser optical systems having double-hinged mechanisms that are swung into the beam path (FIG. 1). It must be ensured in this context that both diaphragm imaging conditions exist as described above.
U.S. Pat. No. 5,684,625 discloses an illumination device having two index positions: the standard range 10×–100×, and the scanning range 1.6×–5×. As the scanning condenser head is swung in, an additional lens is simultaneously swung in by means of a complex mechanical coupling, so that Köhler illumination is maintained for the entire magnification range. An extension into the macro range down to 1× objectives is not technically possible.
JP 9033820A describes an illumination device having three index positions: the standard range 10×–100×, the scanning range 1.6×–5×, and the macro range=1×. With this illumination device, the extension of the magnification range is achieved by way of an additional optical cascade. The complexity and technical risk are thereby increased, however, because of the additional condenser head, the complex pivoting mechanism of the three-head switching system in a very limited space below the specimen stage, and the additional reduction in installation space due to the condenser disk for receiving light rings and DIC prisms for contrasting techniques. The Köhler illumination principle exists for the standard and scanning ranges, and critical illumination for the macro range.
The optical illumination method disclosed in EP 0 841 584 A2 provides two index positions: the standard range 10×–100×, and the scanning range 1×–5×. This expanded objective magnification range is achieved with only one simple switchover below the object stage. The standard range is implemented with a condenser head using the Köhler illumination principle. For the lower magnifications, switching out the condenser head simultaneously switches in an assembly located lower down, so that the overall result is an afocal system which no longer conforms to the Köhler illumination principle. In this configuration, the field diaphragm of the standard range becomes the aperture diaphragm of the scanning and macro range. The aperture diaphragm of the standard range does not, however, take over the function of the field diaphragm for the lower-magnification objectives, or at most can limit the field of view very approximately; this results in inhomogeneously illuminated object fields when the diaphragm is closed. This unsatisfactory type of field limitation represents the greatest disadvantage of this kind of illumination.
The requirements for the condenser system are stringent, since size ranges for objectives from 1× to 100× must be illuminated using the same optical system. These boundary conditions could hitherto be satisfied only by complex and intricate multi-stage condenser systems that have the disadvantages described above.